The present invention relates to microelectromechanical structures, and more particularly to thermally actuated microelectromechanical mirror structures and associated methods.
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are utilized and optical applications which include MEMS light valves and shutters.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. Some MEMS devices are driven by electromagnetic fields, while other micromachined structures are activated by piezoelectric or electrostatic forces. Recently, MEMS devices that are actuated by the controlled thermal expansion of an actuator or other MEMS component have been developed. For example, U.S. patent application Ser. Nos. 08/767,192; 08/936,598, and 08/965,277 which are assigned to MCNC, the assignee of the present invention, describe various types of thermally actuated MEMS devices. The contents of each of these applications are hereby incorporated by reference herein. Thermal actuators as described in these applications comprise arched beams formed from silicon or metallic materials that further arch or otherwise deflect when heated, thereby creating motive force. These applications also describe various types of direct and indirect heating mechanisms for heating the beams to cause further arching. While the thermally-actuated MEMS devices of these applications are described in conjunction with a variety of MEMS structures, such as MEMS relays, valves and the like, these applications do not describe thermally-actuated mirror assemblies.
However, MEMS devices including moveable mirror structures have been developed. Commonly, MEMS moveable mirror devices have been used to redirect electromagnetic energy traveling along a path, typically a light or laser beam. For instance, U.S. patent application Ser. No. 08/719,711, also assigned to MCNC and incorporated by reference herein, describes various types of MEMS devices which can rotate a reflective plate about several axes within a framed structure. While these devices can be used for communications, laser printing, or various other applications, these do not provide laterally moveable mirrors.
Lucas NovaSensor of Fremont, Calif. has also developed a variety of MEMS devices including thermally actuated mirror structures. For example, these mirror structures include a matrix addressable thermally actuated mirror suitable for use in an optical switching array. These mirror structures generally include silicon beams connected to the mirror that conduct electrical current and are deflected by the resulting heat in order to position the mirror. In some of the mirror structures, the mirror is conductive and forms part of the electrical heating circuit. Regardless of the manner in which the structures are actuated, the reflective surfaces of the mirrors are disposed in a plane parallel to the underlying substrate when the device is not actuated and can be moved either in plane or out of plane upon thermal actuation.
While some thermally activated MEMS mirror structures have been developed, it would still be advantageous to develop other types of moveable mirror structures that would be suitable for a wider variety of applications. For instance, moveable mirror structures that have mirrors disposed out of plane relative to both the underlying substrate and the direction of movement provided by the actuator are needed. Further, it would be advantageous to provide a MEMS moveable mirror device that could precisely position a mirror and reliably hold the mirror in position, even after the thermal energy used to position the mirror is removed. The efficiency and performance of MEMS mirror devices in applications involving the precise deflection of multiple narrow beams of electromagnetic radiation could thus be improved. For example, high resolution optical switching arrays could be developed from MEMS mirror devices providing these advantageous attributes.
The present invention provides several embodiments of a moveable microelectromechanical mirror structure that collectively satisfy the above needs and provide several advantageous features. According to the present invention, the moveable MEMS mirror structure includes a thermal actuator and a mirror having a mirrored surface that is disposed out of plane relative to the thermal actuator and to the underlying microelectronic substrate. The MEMS mirror structure provides precise movement of the mirror using the thermal actuator and permits the mirror to be held in a fixed position, even after the thermal actuator is deactivated. Further, MEMS moveable mirror structures may be disposed in an array and individually controlled to serve a variety of switching applications or the like.
In one embodiment, the MEMS moveable mirror structure includes a microelectronic substrate having a first major surface, a microactuator, and a mirror. The microactuator is preferably formed from a single crystal material and is disposed upon the first major surface of the microelectronic substrate. The microactuator is thermally actuated so as to controllably move along a predetermined path that extends substantially parallel to the first major surface of the microelectronic substrate. The mirror is also preferably formed from the single crystal material and is adapted for movement with said microactuator. In particular, the mirror is arranged to move with the microactuator in response to thermal actuation, thus having a non-actuated position and an actuated position. The actuated position can vary accordingly as the microactuator moves along the predetermined path in response to thermal actuation. According to the present invention, the mirror has a mirrored surface disposed out of plane relative to the first major surface of the microelectronic substrate whether in the non-actuated or actuated position.
In one embodiment, the microactuator of the MEMS moveable mirror structure comprises a thermal arched beam actuator. This actuator includes at least two anchors affixed to the microelectronic substrate and at least one thermal arched beam disposed between the anchors. Each thermal arched beam is adapted to arch further and controllably move along the predetermined path in response to the selective application of thermal actuation. The microactuator can optionally include a spring adapted to flex during selective thermal actuation. While the thermal arched beam actuator need only have a single arched beam, the microactuator of the MEMS moveable mirror structure can comprise a plurality of thermal arched beams. In one embodiment, for example, the plurality of thermal arched beams are arrayed to expand in response to thermal actuation and collectively move along the predetermined path. In another embodiment, the plurality of thermal arched beams are arrayed to compress in response to thermal actuation and collectively move along the predetermined path. In any embodiment, the thermal arched beam actuator can include an electrically conductive path disposed through or upon at least part of the thermal arched beams in order to direct the current flow and correspondingly control the heating of the thermal arched beams.
In another embodiment, the microactuator of the MEMS moveable mirror structure comprises at least one thermally actuated composite beam actuator. This actuator includes at least one anchor affixed to the microelectronic substrate and a composite beam extending from the anchor and overlying the first major surface thereof. Each composite beam has a proximal end connected to an anchor, and a distal end adapted to bend so as to move the mirror along the predetermined path in response to selective thermal actuation, as before. Each composite beam also comprises at least two layers that respond or expand differently to thermal actuation. The first and second layers may be formed from materials with different thermal coefficients of expansion, such that the distal end bends toward the layer having the lower thermal coefficient of expansion when thermally actuated. An electrically conductive path encompassing the distal end of the composite beam and having variable electrical resistance is defined by the first and second layers of the composite beam, such that current passing along the conductive path causes thermal actuation of the composite beam. Dual thermally actuated composite beam actuator structures with enhanced linear displacement characteristics are provided, including advantageous interconnecting members, interconnecting structures, and platforms used therewith to carry and correspondingly move the mirror.
One embodiment of the MEMS moveable mirror structure further comprises a mechanical latch affixed to the first major surface of the microelectronic substrate. The mechanical latch is adapted to open in response to thermal actuation so as to receive the microactuator. Further, the mechanical latch is adapted to close when thermal actuation is removed to controllably clamp the microactuator in position once the mirror has moved into the actuated position. Once latched, the microactuator and, therefore, the mirror can be held in place even if the microactuator is no longer actuated. In addition, the mechanical latch is adapted to reopen in response to further thermal actuation to release the microactuator. In another embodiment, an electrostatic latch is provided to clamp the microactuator in position. The electrostatic latch includes an actuator electrode disposed on the microactuator and a substrate electrode disposed on the microelectronic substrate. When a voltage is applied between the electrodes, an electrostatic force is created therebetween to controllably clamp the microactuator in position at any position along the predetermined path of movement.
A further embodiment of the present invention provides a MEMS mirror array including a microelectronic substrate and a plurality of microelectromechanical mirror structures. Each mirror structure comprises a microactuator and mirror as described in the earlier embodiments. One or more of the mirrors within the array can therefore be controllably positioned by selectively thermally actuating the microactuators corresponding to the respective mirrors. For example, the MEMS mirror array can further include an activation matrix having a row activation path and a column activation path operably connected to each moveable mirror structure within the array. Each mirror can thus be controllably positioned through thermal actuation of the respective microactuator by activating the row and column activation paths corresponding to the mirror. As described above, the MEMS mirror array can include a variety of microactuators as well as a spring and a latch, such as a mechanical latch or an electrostatic latch. The MEMS mirror array can also include a source of electromagnetic radiation directed along at least one path intersecting one or more of the mirrors within the array, such that the electromagnetic radiation is redirected by a mirror.
Consequently, the present invention also provides a method of redirecting electromagnetic radiation directed along at least one path using one or more moveable mirror structures. One embodiment of the method comprises the steps of providing at least one source of electromagnetic radiation directed along at least one path, selectively thermally actuating one or more microactuators to controllably move along the predetermined path, controllably moving the mirrors corresponding to the actuated microactuators so as to intersect at least one path of electromagnetic radiation, and redirecting at least one path of electromagnetic radiation intersected by the mirrors. As described above, the mirrors can be clamped in position using the mechanical or electrostatic latches in order to reduce energy consumption.
A method of fabricating an microelectromechanical mirror structure is also provided by the present invention. One embodiment of the method includes the steps of providing a carrier wafer having a first major surface, bonding a single crystal wafer thereto, selectively etching the single crystal wafer to define a mirror having a mirrored surface disposed out of plane relative to the first major surface of the carrier wafer in both actuated and non-actuated positions, and further selectively etching the single crystal material to define a microactuator integral with the mirror. The microactuator is formed with portions released from the carrier wafer so that thermal actuation of the microactuator along the predetermined path parallel to the first major surface of the carrier material will correspondingly move the mirror between the nonactuated and actuated positions. Other embodiments further define the fabrication of the mirror, microactuator, and latches as disclosed herewith.
Although the foregoing invention will be described in some detail, it will be obvious that certain changes and modifications may be practiced within the scope of the invention described herein.